Nickel-chromium alloy and method of making the same

ABSTRACT

A nickel and chromium alloy having a combined wt. % of nickel and chromium of at least 97 wt. %, wherein the chromium accounts for 33 to 50 wt. % of the alloy. The alloy may be provided in strip form and has adequate ductility for the manufacture of various products, such as sheaths for flux cored welding electrodes. A method of making the alloy strip includes forming a powder charge that is 97 to 100 wt. % of nickel and chromium combined and the chromium accounts for 33 to 50 wt. % of the charge, roll compacting the powder charge to form a green strip, sintering the green strip to form a sintered strip, and cold rolling and annealing the sintered strip to form the alloy strip.

FIELD OF THE INVENTION

The invention relates to nickel-chromium alloys and methods of makingnickel-chromium alloys having improved ductility that may be provided instrip form. The strip may be used in a variety of applications includingthe manufacture of sheaths for flux cored welding electrodes.

BACKGROUND OF THE INVENTION

Ni—Cr alloys are used in many applications requiring high temperatureoxidation and corrosion resistance. Ni—Cr alloys, with high chromiumcontent (35%-60%) are used in cast form for applications in power plantsand boilers where resistance to fuels, ashes, and deposits high insulfur and vanadium content is required. Various alloys in strip formare widely used in many diverse applications, including the manufactureof welding consumables which can also be used for overlay coatings.

Conventional means of producing Ni—Cr alloy strip typically includeadding nickel- and chromium-containing melt stocks into a suitablemelting furnace (along with any desired alloying elements and otherelements required to deoxidize and fluidize the melt), melting thecharge, and casting it into an ingot. This ingot may then be processedby hot and cold working into strip, or may be re-melted to purify thecomposition and refine the cast grain structure prior to being hot andcold worked into strip. This strip may then be used as the outer sheathof a flux cored welding consumable, and as such its chemical compositionneeds to be tightly controlled because the elements contained in thestrip will be incorporated into the weld bead or overlay when thewelding consumable is subsequently used.

In general, Ni—Cr alloys, with high Cr content (greater than 30%) madeby conventional means (melting and casting) are difficult to work andtherefore generally not processed into strip form. The ductility of theNi—Cr alloy is inversely proportional to the concentration of chromiumbecause the chromium has a phase structure which is brittle. Ni—Cralloys having a chromium content greater than 30% are generally onlysuited to casting. This occurs because segregation during the castingprocess results in the formation of large volumes of the brittle phasesof chromium within the cast structure of the ingot and reduces theductility of the alloy to the point where the alloy does not havesufficient ductility for working the cast ingot into a strip useable asa sheath in a flux cored welding electrode.

In contrast, welding consumable manufacturers desire to make productscontaining high chromium levels because these high chromium materialscan produce weld beads or overlays that exhibit enhanced corrosionresistance and wear resistance. Thus, a cheaper base material can beoverlaid with a layer of high chromium Ni—Cr alloy so that it will havesufficient corrosion or wear resistance in service. Because of theembrittlement problem with high chromium content Ni—Cr alloys, when awelding consumable manufacturer wishes to produce a product that is ableto deposit a weld bead or overlay of Ni—Cr composition in excess of 33weight percent (wt. %) chromium, the only means possible is to usecommercially available strip for the weld wire sheath with lower than 33wt. % chromium content and add the extra required chromium by blendingchromium powder or a chromium alloy powder with the other constituentsthat are contained within the core of the welding wire. This, of course,adds costs to the manufacture of the welding consumable.

There is therefore a need to provide Ni—Cr alloy strip containingchromium in excess of 33 wt. % chromium balance nickel in strip formhaving adequate ductility and formability, so that it can be used as thesheath in flux cored welding consumables. This would reduce or eliminatethe need to make chromium additions to the core of the welding wire,thereby lowering the cost of making such wire.

SUMMARY OF THE INVENTION

One aspect of the present invention is to provide an alloy in strip formcomprising nickel and chromium having a combined wt. % of nickel andchromium of at least 97 wt. %, wherein the chromium accounts for 33 to50 wt. % of the alloy.

Another aspect of the present invention is to provide an alloycomprising a combined wt. % of nickel and chromium of at least 99.8 wt.%, wherein the chromium accounts for 33 to 50 wt. % of the alloy.

Yet another aspect of the present invention is to provide a Ni—Cr stripmade from an alloy according to the present invention having adequateductility, such that it may be formed into various products, such as asheath for a welding electrode.

Finally, yet another aspect of the present invention is to provide amethod of making an alloy strip according to one embodiment of thepresent invention comprising forming a powder charge, wherein the powdercharge comprises 97 to 100 wt. % of nickel and chromium combined and thechromium accounts for 33 to 50 wt. % of the charge; roll compacting thepowder charge to form a green strip; sintering the green strip to form asintered strip; and cold rolling and annealing the sintered strip toform the alloy strip.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a magnified image of the microstructure of a Ni—Cr alloy stripmade according to one embodiment of the present invention.

DETAILED DESCRIPTION

Generally, an embodiment of the present invention includes an alloy thatmay be made using a roll compaction process. The alloy may comprisenickel and chromium having a combined wt. % of nickel and chromium of atleast 97 wt. %, wherein the chromium accounts for 33 to 50 wt. % of thealloy, more preferably 35 to 50 wt. %, and most preferably 40 to 50 wt.%, and the alloy may comprise less than 3 wt. % of Mn and Si combined.In certain embodiments, the nickel content of the alloy may be at least47 wt. %. The alloy may be provided in strip form having a tensileelongation of at least 30%.

Another embodiment of the present invention includes an alloy comprisingnickel and chromium having a combined wt. % of nickel and chromium of atleast 99.8 wt. %, wherein the chromium accounts for 33 to 50 wt. % ofthe alloy, more preferably 35 to 50 wt. %, and most preferably 40 to 50wt. %, and the alloy may exclude Mn and Si. In certain embodiments, thenickel content of the alloy may be at least 49.8 wt. %.

In yet another embodiment of the present invention, a method of makingan alloy strip comprises forming a powder charge, wherein the powdercharge comprises 97 to 100 wt. % of nickel and chromium combined and thechromium accounts for 33 to 50 wt. % of the charge; roll compacting thepowder charge to form a green strip; sintering the green strip to form asintered strip; and cold rolling and annealing the sintered strip toform the alloy strip. The powder charge may be formed from at least oneof a nickel containing powder and chromium containing powder that is atleast 99.5% pure. In certain embodiments, a sheath for a flux coredwelding electrode may be made by forming the sheath from an alloy madeaccording to the present invention.

As explained above, it is difficult to process Ni—Cr alloys into stripmade by conventional means when the composition contains in excess of 33wt. % chromium due to the presence of brittle phases. In light of this,current practice within the welding consumables manufacturing industryfor making products capable of depositing a weld bead or overlay withmore than 33 wt. % chromium content is to use a strip as the consumablesheath that contains less than 33 wt. % chromium and add chromium to theflux core to provide the additionally required chromium to the product.The present invention eliminates or reduces the necessity of additionalchromium to the flux core since the strip produced can contain in excessof 33 wt. % chromium and still exhibit sufficient ductility andformability to be successfully processed into sheaths for flux coredwelding electrodes.

To manufacture a Ni—Cr alloy strip having a chromium content which maybe used in a welding consumable and have the ductility necessary to formthe alloy into a sheath for a welding consumable, a process has beendiscovered within the scope of the invention that is able to provide aformable Ni—Cr alloy containing chromium in excess of 33 wt. %,preferably 33 to 50 wt. % chromium, more preferably 35 to 50 wt. %chromium, and most preferably 40 to 50 wt. % chromium. The combined wt.% of the nickel and chromium in the alloy may be at least 97%,preferably at least 98%, more preferably at least 99%, and mostpreferably at least 99.8%. The resulting alloy has ductilitycharacteristics which allow it to be drawn and formed into wire orsheaths for welding consumables. The Ni—Cr alloy according to thepresent invention has improved ductility and is not negatively affectedby the brittle phases associated with higher chromium levels whencompared to conventional Ni—Cr alloys having similar Cr content madeusing the casting process.

As used herein throughout the specification and the claims, the term“strip” includes all materials commonly known in the industry as sheet,strip, or foil that is less than 0.050 inches in thickness.

Preferably, “sufficient ductility” is the extent to which the alloy maybe formed without fracture. The extent to which mechanically formingstresses an alloy sheet or strip when making a welding consumable iswell-known in the art. Thus, the process of the present inventionenables the production of a Ni—Cr alloy having a high chromium contentand sufficient ductility to endure the forming processes associated withthe production of welding consumables. “Sufficient ductility” as usedherein is defined as tensile elongation as determined using ASTM E8, thestandard test method for tension testing of metallic materials and is atleast 30%, more preferably at least 40%.

To manufacture a Ni—Cr alloy strip with a chromium content in excess of33 wt. %, more preferably in excess of 35 wt. %, and most preferably inexcess of 40 wt. %, that has sufficient ductility, so that it may beformed into a flux cored welding consumable, the present inventionincludes a process that eliminates melting. Because no melting isrequired to make the Ni—Cr alloy strip, there is no cast structurecontaining large grains of brittle phases even though the resultingstrip will have a chromium content in excess of 33 wt. %.

An additional advantage of the present invention is that since nomelting takes place, there is no need to add extra elements to thecomposition to facilitate the melting process (such as deoxidants orfluidizers) and there will be little to no loss of volatile alloyingelements as can occur during melting. Therefore, in one embodiment ofthe present invention, an alloy is provided consisting essentially ofnickel and chromium having a combined wt. % of nickel and chromium of atleast 97 wt. %, wherein the chromium accounts for 33 to 50 wt. % of thealloy. Additional alloying elements other than nickel and chromium maybe present in the alloy, for example, in trace amounts, but theadditional alloying elements are not needed to obtain a strip of thealloy having a tensile elongation of at least 30%. Nickel-based alloysmade by conventional melting and hot & cold rolling techniques typicallyhave a Mn and Si content in the 0.5 to 2.0% range. In certainembodiments of the present invention, a Ni—Cr alloy is provided that isessentially void of Mn and Si. In a preferred embodiment of theinvention, a Ni—Cr alloy is provided that may contain less than 3%, morepreferably less than 1.5%, of Mn and Si combined. In some embodiments,the Ni—Cr alloy of the invention may comprise essentially 0% of Mnand/or Si. As used herein, “essentially 0%” means less than 100 ppm.

A process according to the present invention utilizes powder metallurgy,specifically roll compaction, and includes blending nickel and chromiumcontaining powders in a ratio to make the desired Ni—Cr alloycomposition, consolidating the powders into a green strip via rollcompaction, sintering the green strip to increase its density andstrength and homogenization of the alloying elements, followed by coldrolling and annealing the strip to a final thickness.

A “green” strip as used herein throughout the specification and theclaims means a metal strip produced by roll compaction which has not yetbeen treated to strengthen the material by sintering. Following rollcompaction, the green strip may be sintered under an atmospherecontaining hydrogen to improve the strength and reduce the oxygencontent of the strip. The sintered strip may then be mechanically worked(cold rolling). As used herein throughout the specification and theclaims, the term “cold rolling” means mechanically working the stripbelow the recrystallization temperature of the material.

According to various embodiments of the invention, intermediaterecrystallization anneals may be carried out as required between rollingcycles. The densification of the strip occurs during the sintering, coldrolling, and the recrystallization anneals. The final density of thematerial, or a value close to its theoretical density, is achieved afterthe cold rolling operations.

In one embodiment of the present invention, nickel and chromium powdermay be combined to form the desired alloy composition. It is preferredto use high purity metal powders that are 99.5% pure. As used herein,“99.5% pure” means at least 99.5 wt. % of the powder comprises nickel orchromium. In another embodiment, the metal powders may include asuitable mixed nickel-chromium powder to which a high purity nickel orchromium powder may be added to achieve the desired final weight percentof nickel and chromium in the alloy. When using powders of differentconstituents, the powders should be well mixed to insure homogeneity ofthe powder charge. In order to obtain the desired powder properties forroll compaction, these properties being apparent density, flow, andconsolidation characteristics, along with the properties of theresulting green strip, the average particle size of the powders shouldbe less than about 100 microns.

Other components known in the industry as additives or binders, whichwill preferably volatilize during subsequent processing, may be added tothe powder charge to form a blend. Examples of these addedcomponents/additives would be dispersants, plasticizers, and sinteringaids. Other known expedients may also be added for the purpose ofaltering the flow characteristics and the consolidation behavior of thepowders in the blend. Suitable additives used for altering thecharacteristics of powders are well known in the art of powdermetallurgy and include, for example, long chain fatty acids such asstearic acid, cellulose derivatives, organic colloids, salicylic acid,camphor, paraffin etc. Preferably, the additives used in the blendshould be kept at amounts lower than 2 wt % of the blend. The powdermaterials and additives may be combined using any suitable techniqueknown in the art. For example, a V-cone blender may be used.

Additional nominal alloying elements for the Ni—Cr alloys according tothe present invention may be selected and incorporated based on thedesired properties of the final strip, such as the mechanicalproperties, e.g. yield strength, ultimate tensile strength, and %elongation, etc. When incorporating nominal alloying elements, the Ni—Cralloy strip made according to various embodiments of the presentinvention may include up to 3 wt % of the nominal alloying elements.

Upon adding any additives to obtain a powder blend, the material maythen be roll compacted to form a green strip having a desired thickness.The powder material is preferably roll compacted by delivering thepowder charge such that the powder cascades vertically between twohorizontally opposed rolls with the powder fed into the roll nip in auniform way.

The density and dimensions of the green strip is determined primarily bythe physical properties of the powder and spacing provided between thehorizontally opposed rolls as well as the forces applied by the rolls.The preferred thickness of the green strip is 0.050″ to 0.200″, morepreferably 0.060″ to 0.150″. Because the initial green strip issubstantially thinner than the Ni—Cr ingots produced by conventionalprocesses, embodiments of the present invention may require less work,and as a result, less processing time, to provide a strip having thedesired thickness upon finishing. It is preferred that the resultinggreen strip has a density that is 50% to 90% of theoretical density,more preferably 60% to 90% of theoretical density.

According to an embodiment of the present invention, a green strip maybe provided by roll compacting as described above and followed bysintering. Sintering requires heating the green strip under a controlledatmosphere for a period of time. The sintering process reduces theoxygen content of the strip as well as provides inter-particle bondingand an increase in density, so that the strength of the resulting stripis significantly increased. It is preferred that sintering occur under agaseous atmosphere comprising at least 10% hydrogen, more preferably 25%to 100% of hydrogen. Sintering may also occur under vacuum or partialpressure of an inert gas or more preferably under partial pressure ofhydrogen. Sintering occurs at temperatures of 1000° C. to 1350° C., morepreferably from 1100° C. to 1350° C., most preferably from 1150° C. to1250° C. The sintering process may last from 1 to 12 hours when highertemperatures are used and 12 to 48 hours at the lower sinteringtemperatures.

In order to further reduce the thickness of the sintered strip to alighter gauge material, embodiments of the present invention include aprocess comprising a combination of cold rolling and annealing thesintered strip to form the final strip. In embodiments of the presentinventive method, it is preferred that the cold rolling steps occurs atroom temperature. It is also preferred that cold rolling occurs underconditions which minimize oxidation of the sintered strip. Because thetemperatures are generally low enough that oxidation is not a concern,cold rolling may occur under an oxygen containing atmosphere, such asair.

Cold rolling comprises working the material in order to reduce thestrip's thickness. The strip may be passed one or more times through arolling process. The total number of passes constituting one coldrolling cycle. According to an embodiment of the present invention, thestrip thickness may be reduced 1% to 30% per pass, preferably 5% to 20%per pass, by cold rolling. The total reduction per rolling cycle ispreferably 20% to 50%, more preferably 30% to 40%.

Embodiments of the present inventive method may also includerecrystallization annealing steps. Recrystallization annealing iscarried out at a temperature above the recrystallization temperature ofthe material in order to reduce its strength and hardness and isaccompanied by changes in the microstructure. According to variousembodiments of the present invention, the recrystallization annealoccurs at a temperature from 1900° F. to 2300° F. (about 1038° C. to1260° C.). The total time required for a recrystallization anneal may beshorter if higher temperatures are used. Similar to sintering, annealingpreferably occurs under a gaseous atmosphere comprising hydrogen and/orunder partial pressure of hydrogen, or the recrystallization annealingmay occur under vacuum or inert gas.

Embodiments of the present invention may include a plurality of coldrolling cycles with annealing steps occurring between each cold rollingcycle. Again, each cold rolling cycle may include multiple passes.Following the final cold rolling cycle, the Ni—Cr alloy strip has athickness that is preferably 35% or less of the thickness of theoriginal green strip, i.e. reduction of a green strip according to anembodiment of the present invention may require about 65% reduction ormore to reach the target thickness.

EXAMPLES

In order that the invention may be more fully understood the followingExamples are provided by way of illustration only.

Example 1

Powders containing 1,474 grams of nickel, 794 grams of chromium, and 39grams of Fe—Si alloy were blended together in a V-cone blender. Theblend was fed vertically down into a roll compaction mill to produce agreen strip. The resulting green strip was sintered in a hydrogenatmosphere furnace at 2200° F. The strip thickness at this point was0.096″. The nominal composition of the sintered strip was 63.9% Ni,34.4% Cr, 0.4 Fe %, and 1.3% Si (wt. percents).

The strip was subsequently rolled from 0.096″ to 0.050″, then from0.050″ to 0.030″, and finally from 0.030″ to 0.020″ with intermediateanneals at 1900° F. for 10 minutes. The final 0.020″ thick strip wasslit to have a width of 0.500″, annealed at 2100° F., and tested for itsmechanical properties using standard ASTM E8 test procedures. Themeasured values for the mechanical properties were the following:

Yield Strength: 25.3 ksi

Ultimate Tensile Strength: 83.2 ksi

% Elongation: 46.6

A magnified image of the microstructure of the strip made in Example 1is provided as FIG. 1.

Example 2

Powders containing 1361 grams of nickel, 907 grams of chromium, and 39grams of Fe—Si alloy were blended together in a V-cone blender. Theblend was fed vertically down into a roll compaction mill to produce agreen strip. The resulting green strip was sintered in a hydrogenatmosphere furnace at temperature of 2200° F. The strip thickness atthis point was 0.108″ thick. The nominal composition of the sinteredstrip was 59.0% Ni, 39.3% Cr, 0.4% Fe, and 1.3% Si (wt. percents).

The strip was rolled from 0.108″ to 0.050″, from 0.050″ to 0.030″, andfinally from 0.030″ to 0.020″ with intermediate anneals at 1900° F. for10 minutes. The final 0.020″ thick strip was slit to have a width of0.500″, annealed at 2100° F., and tested for its mechanical propertiesusing standard ASTM E8 test procedures. The measured values for themechanical properties were as follows:

Yield Strength: 45.9 ksi

Ultimate Tensile Strength: 101.2 ksi

% Elongation: 42.8

While preferred embodiments of the invention have been shown anddescribed herein, it will be understood that such embodiments areprovided by way of example only. Numerous variations, changes, andsubstitutions may occur to those skilled in the art without departingfrom the spirit of the invention. Accordingly, it is intended that theappended claims cover all such variations that fall within the spiritand scope of the invention.

1-22. (canceled)
 23. An alloy comprising nickel and chromium having acombined wt. % of nickel and chromium of at least 97 wt. %, wherein thechromium accounts for 33 to 50 wt. % of the alloy, wherein the alloy isprovided in strip form, and wherein the strip has a tensile elongationas determined using ASTM E8 of at least 30%.
 24. The alloy of claim 23,wherein a roll compaction process is used to produce the strip.
 25. Thealloy of claim 23, wherein the chromium accounts for 35 to 50 wt. % ofthe alloy.
 26. The alloy of claim 23, wherein the chromium accounts for40 to 50 wt. % of the alloy.
 27. The alloy of claim 23, wherein thealloy comprises less than 3 wt. % of Mn and Si combined.
 28. The alloyof claim 23, wherein the nickel accounts for at least 47 wt. % of thealloy.
 29. A welding electrode sheath comprising the alloy of claim 23.30. An alloy comprising nickel and chromium having a combined wt. % ofnickel and chromium of at least 99.8 wt. %, wherein the chromiumaccounts for 33 to 50 wt. % of the alloy, wherein the alloy is providedin strip form, and wherein the strip has a tensile elongation asdetermined using ASTM E8 of at least 30%.
 31. The alloy of claim 30,wherein a roll compaction process is used to produce the strip.
 32. Thealloy of claim 30, wherein the chromium accounts for 35 to 50 wt. % ofthe alloy.
 33. The alloy of claim 30, wherein the chromium accounts for40 to 50 wt. % of the alloy.
 34. The alloy of claim 30, wherein thealloy comprises essentially 0% Mn and Si.
 35. The alloy of claim 30,wherein the nickel accounts for at least 49.8 wt. % of the alloy.
 36. Awelding electrode sheath comprising the alloy of claim
 30. 37. A methodof making an alloy strip comprising: forming a powder charge, whereinthe powder charge comprises 97 to 100 wt. % of nickel and chromiumcombined and the chromium accounts for 33 to 50 wt. % of the charge;roll compacting the powder charge to form a green strip; sintering thegreen strip to form a sintered strip; and cold rolling and annealing thesintered strip to form the alloy strip
 38. The method of claim 37,wherein the powder charge is formed from at least one of a nickelcontaining powder and chromium containing powder that is at least 99.5%pure.
 39. A method of making a sheath for a flux cored welding electrodecomprising forming the sheath from the alloy made according to claim 37.40. A method of making an alloy strip comprising: forming a powdercharge, wherein the powder charge comprises at least 99.8 wt. % ofnickel and chromium combined and the chromium accounts for 33 to 50 wt.% of the charge; roll compacting the powder charge to form a greenstrip; sintering the green strip to form a sintered strip; and coldrolling and annealing the sintered strip to form the alloy strip. 41.The method of claim 40, wherein the powder charge is formed from atleast one of a nickel containing powder and chromium containing powderthat is at least 99.8% pure.
 42. A method of making a sheath for a fluxcored welding electrode comprising forming the sheath from the alloymade according to claim 40.